Interpretive Summary: Growth is controlled by a number of interacting hormones in all animals including fish. The hormones and their target tissues are collectively called the growth axis. Understanding these hormones's actions on the growth axis is key to controlling and improving growth in farmed animals. A group of proteins known to influence the actions of growth axis hormones are called the IGFBP's or insulin-like growth factor binding proteins. These IGFBPs are found in blood and change in abundance depending on whether conditions favor growth or not. In most fish, injection with growth hormone promotes growth and IGFBP-3, one of this family of IGFBPs increases in abundance. However, in channel catfish growth hormone treatment leads to a reduction in IGFBP-3. The significance of this finding is not fully understood, but it has been previously shown that growth hormone treatment in catfish has some unusual affects compared to other fish. It is possible that the unusual effects of GH treatment are related to the elevation in IGFBP-3 unique to catfish.

Technical Abstract:
Vertebrate growth is principally controlled by growth hormone (GH) and its intermediary, Insulin-like growth factor-I (IGF-I). The actions of IGF-I are modified by high-affinity serum binding proteins called Insulin-like growth factor binding proteins (IGFBPs). Channel catfish exhibit atypical response (increased % body fat and reduced % protein) to GH treatment in comparison to other teleost. Among possible explanations for this atypical response to GH treatment is unusual regulation of the IGFBP¿s. As for IGFBPs, there has been one report of a single 33 kDa binding protein in this species. To examine this, GH levels were altered and the type and regulation of IGFBPs in this species were examined. We have identified 5 IGFBPs (19, 35, 44, 47 and 80 kDa) in catfish plasma that are differentially altered by experimental treatment and genetic lineage. Most notable were the regulation of the 19, 35, 44 and 47 kDa IGFBPs. Levels of the 19 kDa IGFBP were elevated in catfish exposed to a higher environmental temperature (26°C versus 21°). In most vertebrates, treatment with GH increases levels of IGFBP-3 (~40-50kDa). In all of the catfish genetic strains tested, elevated GH levels resulted in reductions of the 35, 44 and 47 kDa IGFBPs, suggesting that the abnormal response in GH treatment is not attributed to the use of heterologous GH. This negative response of the 35-47 kDa IGFBPs to the GH has not been reported in any other teleost or vertebrate and may be responsible for the atypical physiological responses of channel catfish to GH treatment.